Recently, hybrid vehicles and electric vehicles have attracted great attention as environment-friendly vehicles. Some hybrid vehicles are now commercially available.
The hybrid vehicle includes, as a power source, a DC power supply, an inverter and a motor driven by the inverter, in addition to the conventional engine. Specifically, the engine is driven to generate power while DC voltage from the DC power supply is converted into AC voltage by the inverter to rotate the motor by the AC voltage and accordingly generate power. The electric vehicle includes, as a power source, a DC power supply, an inverter and a motor driven by the inverter.
Some hybrid or electric vehicles are designed to boost DC voltage from the DC power supply by an up-converter and to supply the boosted DC voltage to the inverter driving the motor.
Japanese Patent Laying-Open No. 2-308935 discloses an electric device 300 shown in FIG. 13. This electric device 300 is mounted on a hybrid vehicle. Referring to FIG. 13, electric device 300 includes a DC power supply 310, a bypass line 311, a relay 312, a boost chopper 320, a capacitor 326, an inverter 330, an electric device body 350, and a field magnet controller 360.
Bypass line 311 and relay 312 are connected in series between a power supply line and the positive electrode of DC power supply 310.
Boost chopper 320 includes a reactor 321, MOS transistors 322, 323, and diodes 324, 325. Reactor 321 has one end connected to the power supply line of DC power supply 310 and the other end connected to the intermediate point between MOS transistor 322 and MOS transistor 323. MOS transistors 322 and 323 are connected in series between the power supply line and a ground line. MOS transistor 322 has its drain connected to the power supply line. MOS transistor 323 has its source connected to the ground line. Diodes 324, 325 are each connected between the source and drain of the corresponding one of MOS transistors 322, 323 for allowing current to flow from the source side to the drain side.
Inverter 330 is constituted of a U-phase arm 343, a V-phase arm 344 and a W-phase arm 345. U-phase arm 343, V-phase arm 344 and W-phase arm 345 are connected in parallel between the power supply line and the ground line.
U-phase arm 343 is formed of MOS transistors 331 and 332 connected in series. V-phase arm 344 is formed of MOS transistors 333 and 334 connected in series. W-phase arm 345 is formed of MOS transistors 335 and 336 connected in series. Diodes 337-342 are each connected between the source and drain of the corresponding one of MOS transistors 331-336 for allowing current to flow from the source side to the drain side.
Electric device body 350 includes three phase coils and serves as a power generator and a motor for an engine. The U, V, W phase arms of inverter 330 have their respective intermediate points connected to the respective ends of the U, V, W phase coils of electric device body 350. The other end of the U-phase coil is connected to the intermediate point between MOS transistors 331 and 332. The other end of the V-phase coil is connected to the intermediate point between MOS transistors 333 and 334. The other end of the W-phase coil is connected to the intermediate point between MOS transistors 335 and 336.
Field magnet controller 360 includes a diode 361, an NPN transistor 362 and a base amplifier 363. Diode 361 is connected between the positive terminal F+ of the field coil of electric device body 350 and the collector of NPN transistor 362. NPN transistor 362 is connected between the negative terminal F− of the field coil and the ground line for receiving at its base a voltage from base amplifier 363. Base amplifier 363 is responsive to a control signal from a control device (not shown) to output a prescribed voltage to the base of NPN transistor 362 for turning on/off NPN transistor 362.
DC power supply 310 outputs a DC voltage. When relay 312 is turned on by the control signal from the control device (not shown), bypass line 311 supplies the voltage on both ends of capacitor 326 to DC power supply 310. Boost chopper 320 has its MOS transistors 322, 323 turned on/off by the control device (not shown) and boosts the DC voltage supplied from DC power supply 310 to provide an output voltage to inverter 330. Boost chopper 320 also down-converts the DC voltage generated by electric device body 350 and converted by inverter 330 to charge DC power supply 310, at the time of regenerative braking of the hybrid vehicle including electric device 300.
Capacitor 326 smoothes the DC voltage supplied from boost chopper 320 and supplies the smoothed DC voltage to inverter 330.
Inverter 330 receives the DC voltage from capacitor 326 to convert the DC voltage to an AC voltage based on the control from the control device (not shown) and drives electric device body 350 as a driving motor. Field magnet controller 360 allows current to flow in the field coil in accordance with the time period during which NPN transistor 362 is turned on. Electric device body 350 is therefore driven as a driving motor to generate torque specified by a torque command value. In regenerative braking of the hybrid vehicle including electric device 300, inverter 330 also converts an AC voltage generated by electric device body 350 to a DC voltage based on the control from the control device and supplies the converted DC voltage to boost chopper 320 through capacitor 326.
In electric device 300, a failure in boost chopper 320 is detected by detecting that the output voltage of boost chopper 320 becomes lower than a reference value. When the failure in boost chopper 320 is detected, relay 312 is turned on by the control signal from the control device, and bypass line 311 directly supplies the voltage on both ends of capacitor 326 to DC power supply 310.
In electric device 300 disclosed in Japanese Patent Laying-Open No. 2-308935, however, when boost chopper 320 fails, the voltage on both ends of capacitor 326 is supplied to DC power supply 310 without being down-converted. Therefore, if a large amount of power is generated by electric device body 350, a high voltage will be applied to both ends of capacitor 326, resulting in that the withstand voltage performance of capacitor 326 must be improved, thereby increasing costs.